This has nothing to do with the English cognate "amortization" in either of its senses (i.e., neither asset depreciation nor loan repayment). No, what this is referring to is an "amortización de acciones" in a reduction of capital stock, what we call a redemption of shares or stock buyback in English. I've seen this hundreds of times in corporate bylaws and minutes of shareholder meetings from Mexico, so I'm positive that's the meaning. My only hesitation is exactly how to word this. Here, I'm adding the word "distribution" because I believe that is what the "prorrateo" part is referring to. I understand it to mean the basis of proportionality of how the buyback is to distribute the capital to the shareholders.

"amortización de acciones - repurchase of shares of stock by a corporation, redemption of stock, stock buyback, share buyback"
Javier F. Becerra, Dictionary of Mexican Legal Terminology, 2nd ed., p 84.

"26 U.S. Code § 302. Distributions in redemption of stock
...
(4)Redemption may be pro rata
Whether or not a redemption meets the requirements of subparagraphs (A) and (B) of paragraph (2) shall be determined without regard to whether or not the redemption is pro rata with respect to all of the shareholders of the corporation."
https://www.law.cornell.edu/uscode/text/26/302